1. Field of the Invention
The present invention relates to a cooling drum for a twin-drum type continuous casting system or a single-drum type continuous casting system, and particularly relates to a method for manufacturing such cooling drum.
2. Description of the Prior Art
Heretofore, in a system for continuously casting a band-shaped cast piece with a single drum or twin drums, various structures of a cooling drum which take prevention of thermal deformation into consideration have been proposed. As one example of such cooling drums, a structure shown in FIG. 6 was disclosed in Laid-Open Japanese Patent Specification No. 3-169461 entitled "Rolls for a system for continuously casting with a single roll or between twin rolls". In this roll, a central portion of a sleeve 7 coming into contact with molten metal is mechanically restrained with respect to a core 6 by means of a side plate 4 and an annular clamp member 3, and the core 6 is fixedly secured to a shaft 2 via a hub 1. The sleeve 7 is cooled by making coolant flow through the inside of the sleeve 7 and the core 6 as shown by arrows in FIG. 6.
In the above-described roll, since the sleeve 7 is mechanically restrained by the core 6, thermal deformation at a position remote from the restrained portion is large, and the magnitude of the thermal deformation increases as a casting time elapses. More particularly, as thermal strain of the sleeve 7 arises in excess of yielding strain, fastening strain between the sleeve 7 and the core 6 is lowered. In addition, due to thermal elongation of the sleeve 7, wear and abrasion of fitting surfaces of the sleeve 7 and the core 6 are caused by slipping therebetween, hence a fastening force is gradually relaxed, and eventually a gap clearance is produced between them.
Consequently, there was a shortcoming that the magnitude of thermal deformation of the cooling roll, which determines a cast piece configuration, would become large as casting time elapses. Working time of a cooling drum was several minutes in the case where the sleeve 7 is made of material having a low thermal conductivity such as, for example, steel, and even in the case of employing material having a high thermal conductivity such as copper alloys, it was several hours at maximum. There was a shortcoming that at a time close to this limit time, thermal deformation exceeded 1000 .mu.m and a distribution of a crown of a cast piece also exceeded .+-.50 .mu.m.